1. Field of the Invention
This invention relates generally to digital subscriber line (DSL) solutions. More particularly, it relates to a method and technique for adapting a bandwidth of a discrete multi-tone (DMT) DSL solutions.
2. Background
Digital subscriber line (DSL) technology transforms inexpensive copper phone lines into high speed, high value data service lines. DSL refers to a group of digital data services which support data speeds from 128 Kbps to 7 Mbps over standard copper phone lines. The first true DSL was ISDN and while that service has become popular, the limited bandwidth options make it less appealing than the newer high speed alternatives that have been developed.
DSL was originally designed to allow regular phone services even in the event of power outages-in what is termed “lifeline POTs” or “Plain Old Telephone Service.” This feature is still available in the asymmetric DSL (ADSL) and rate adaptive DSL (RADSL) variations of DSL. In fact, with ADSL and RADSL, users get the benefits of using not only a single pair of wiring, but get both high speed digital data services and their regular lifeline telephone service over that wiring.
Traditional analog voice services require 300 Hz to 3,400 Hz of bandwidth 521 on a local loop of copper wiring (i.e., the telephone line) between traditional central office switches and customer premises, as shown in FIG. 7. These same wires are, however, capable of carrying information at much higher rates when modern digital signal processing technologies are used. The explosive growth in Internet access, as well as remote LAN access and telecommuting has resulted in a high demand for faster data services. DSL technologies utilize a bandwidth 523 of up to 1.2 MHz (over 300 times the bandwidth of an analog phone call) as shown in FIG. 8, and allows data speeds of over 7 Mbps.
As its name implies, ADSL transmits an asymmetric data stream, with up to 7 Mbps downstream bandwidth (to the subscriber) and only up to 1 Mbps upstream bandwidth. The reason for this asymmetry has less to do with transmission technology than with the telephone cabling. Twisted pair telephone wires are bundled together in large cables. Fifty pair to a cable is a typical configuration towards the subscriber, but cables coming out of a central office may have hundreds or even thousands of pairs bundled together. An individual line from a central office to a subscriber is spliced together from many cable sections as they fan out from the central office. Twisted pair wiring was designed to minimize the interference of signals from one cable to another, but the process is not perfect. Signals do interfere with one another as frequencies and the length of line increase. In fact, if you try to send symmetric signals in many pairs within a cable, you significantly limit the data rate and length of line that you can attain.
Asymmetric solutions are targeted primarily at individual Internet subscribers who receive more information than they send. Businesses typically host web servers, requiring high-speed Internet bandwidth in both directions.
Two line coding schemes are possible with ADSL: Discrete Multi-Tone (DMT) and Carrierless Amplitude and Phase (CAP) modulation. Although the CAP version has been more widely deployed in trials, DMT is the version approved by ANSI's Working Group T1 E1.4 as the industry standard.
ADSL has two significant advantages. It is the fastest DSL technology that supports the maximum distance in the local loop. Moreover, it supports lifeline or Plain Old Telephone Service (POTS).
With ADSL, data and wireline POTS are provided as independent channels on a single line. Symmetric DSL modulation schemes require a separate voice line—two lines total—to provide both services. This is not a problem in most newer buildings which are usually wired for at least two lines, but ADSL does offer a significant edge in older houses and apartments served by a single line. These two advantages make ADSL the favored long-term solution among carriers and service providers addressing the consumer market.
With ADSL, both upstream and downstream speeds vary with distance. ADSL speeds can vary greatly based on a number of conditions. In areas where there is a large variance in the length of the local loop (distance from the subscriber to the central office), the gauge of the wire, and the condition of the line, it becomes difficult to determine what speeds should be provisioned over each line. It is for these reasons that Rate Adaptive ADSL (RADSL) was developed.
Rate Adaptive ADSL allows automatic, or provider specified, adjustment of the speed on the line. Rate Adaptive Asymmetric Digital Subscriber Line (RADSL) offers a downstream (from the central office or central site to residence) data rate of up to 7.0 Mbps and an upstream (from residence to the central office) speed to 1.0 Mbps. Some of the advantages of RADSL are reduced loop qualification efforts, maximized service coverage, a single product serves multiple applications, simplified deployment, reduced product inventory requirements, adaptability of data rate to changing loop conditions, the availability of bandwidth-based service offerings, and the simplification of service issues due to automatic rate adaptation.
FIG. 9 shows a typical RADSL configuration including a RADSL modem 400 at a subscriber's site. RADSL provides a solution most suitable for low-cost, high speed Internet applications.
Like ADSL, RADSL can use either Carrierless Amplitude Phase (CAP) modulation or Discrete Multi-Tone (DMT) modulation. RADSL technology automatically adjusts line speed based on a series of periodic tests that determine the maximum speed possible on a particular line. Since RADSL accommodates the maximum speed available across a particular line, much of the effort and/or guesswork can be taken out of provisioning ADSL. As with ADSL, RADSL supports both high-speed data and lifeline POTS service.
The primary difference between the RADSL-CAP and RADSL-DMT line cards is in the modulation technique used. CAP treats the entire frequency spectrum as a single channel and optimizes the data rate over that channel. DMT divides the bandwidth into sub-channels and optimizes the data rate for each sub-channel. CAP has been tested longer than DMT and is more widely deployed and used, but DMT has been accepted as the standard by the American National Standards Institute (ANSI) and the Telecommunications Standards Institute (ETSI).
Carrierless Amplitude Phase (CAP) modulation divides the spectrum into three parts; the voice band 510, the upstream communications band 512, and the downstream communications band 514, as shown in FIG. 10. The lower 4 Khz of bandwidth is the band utilized for regular analog voice transmission. Frequencies starting at 26 KHz are used for upstream data communications, and frequencies above 240 KHz are used for downstream data transmission.
The relevant standards committees (i.e., ANSI and ETSI) have approved Discrete Multi-Tone (DMT) technology for implementing broadband copper local loops to the home, and this same technology can be used with any telephone grade twisted pair copper wiring. The DMT technique breaks up the available bandwidth into multiple subchannels, and then modulates each band. Just as is done in CAP, the lower end of the spectrum is left alone for carrying the regular analog phone service. In ADSL DMT-systems, the downstream channels from 26 KHz to 1.2 MHz are divided into 256 4 KHz wide tones. The upstream channels spanning 26 KHz to 138 KHz frequencies are divided into 32 subchannels 613, as shown in FIG. 11. Each subchannel is used as a carrier with bit and power allocations according to the signal to noise ratio characteristics of the subchannel. Thus, the link transmission is optimized by running each of the subchannels at best possible data rates.
The received signal spectrum is broken down into 256 4 KHz bands using digital signal processing techniques after A/D conversion.
In the realm of heavy digital subscriber line (DSL) solutions, there are situations where the interference level relative to the useful signal is such that after applying the programmable gain to the received signal, the analog-to-digital converter input is primarily dominated by interference. Important to the principles of the present invention (as will be discussed) are both interference in the digital domain corrupting digital data as well as interference which may overload analog circuits before an analog-to-digital converter.
Discrete Multi-tone (DMT) modulation is known to offer the advantage of selecting tones with best signal to noise ratios (SNR) and leaving out tones affected by interference. However, there are situations where the interference level relative to the useful signal is still such that after applying the programmable gain to the received signal, the analog-to-digital converter input is primarily dominated by the interference, thereby causing deterioration of receiver performance.
One of the major sources of interference for DMT systems is AM radio interference 599, as shown in FIG. 12. AM radio signals span from about 540 KHz up to 1.6 MHz.
There are typical situations where AM radio interference can be particularly detrimental to a modem's performance. For instance, telephone line loops having unbalanced bridged taps or unbalanced home wiring may cause susceptibility to AM interference. In this case, AM interference results in a strong interference signal in the modem receiver which cannot be totally eliminated by conventional common mode rejection filters.
Other examples include long telephone line loops with high insertion loss, and telephone lines picking up AM radio interference coupled after line insertion loss. In such cases, a high PGA gain is required before analog-to-digital conversion to properly quantize the desired signal. But the presence of the large AM radio signal may overload the analog-to-digital converter if the PGA gain is high.
Other interference sources such as T1 crosstalk also tend to have higher spectral density at higher frequencies.
Ordinarily, the transmitted energy from an AM broadcast station that enters a telephone wire pair exists as a common mode signal. Many safeguards exist to protect a DSL modem from such a signal. However, on occasion, the coupled energy is very strong or may become a differential signal. A very strong AM radio common mode signal may cause overload of a DSL analog front end. Overload can cause the loss of 100 or more DMT carriers. On the other hand, a differential AM radio signal on the digital subscriber line makes the bandwidth occupied by the AM interference unusable for DMT ADSL reception. Each differential interference signal from an AM radio station will cause the loss of 4 to 5 DMT carriers.
All modulation techniques discussed, including all types of DSL techniques, are by way of example only, and should not serve to limit the following described invention in any way.